With resumption of the strategic arms talks today, both major US/Soviet arms parleys are running again in Geneva. And with his newest START proposal - a ''build-down'' or gradual trimming of warheads and missiles by replacement - President Reagan has matched his recent intermediate nuclear force (INF) proposals to show flexibility on both fronts.
Very cautious optimism is probably the most accurate way to assess what might come of the latest American initiatives.
The Reagan build-down - subtracting two old land-based warheads for every new one such as the planned MX, or three submarine missile warheads for two new ones - is on its face a significant and reasonable compromise. The goal would be to reach a 5000-warhead level on each side, deleting more than 2000 warheads from each current arsenal. This approach, the administration argues, is superior to a nuclear freeze - a proposal the Soviets made again at the United Nations this week - because it would directly lead to lower levels. Also, one of the US arms aims is to modernize its missile force, proceeding with the MX missile. A build-down would permit that. A freeze would not. However, just what the 5000 -warhead goal would mean in actual firepower isn't yet clear.
The build-down idea has powerful advocates in the US Congress. Admittedly it was put forward now in greater detail to secure Congressional support for the MX , and to dampen domestic US enthusiasm for a nuclear freeze. Including a member of the President's Commission on Strategic Forces in a special build-down working group at Geneva should further reassure the build-down advocates that their idea - not a favorite in the Defense Department and other reaches of the administration - would get a serious hearing.
The iffy side of all this comes from the basic question of whether the Soviets are so miffed by the public flaying they've received over the Korean Airlines incident that they will want to cool off any new Reagan proposal for a while. They could calculate that choking off START progress, by declining to join any working group study of a build-down, would build up pressure on the INF front. Their goal there is to prevent deployment in Europe of new US cruise and Pershing II missiles as scheduled by the end of this year, pending any INF agreement.
The Soviets clearly have been hunkering down, possibly to show a lack of progress with the US on all fronts. Some arms analysts argue the new proposals, involving complex formulas, might put more freight on the START rails than the negotiations can handle at the moment. Separate pieces of the build-down proposal have been assigned to a Geneva working group and to the principal negotiating venue - an unwieldy arrangement. And the status of the build-down within the Reagan administration itself is by no means settled.
Meanwhile, talks on the European intermediate range nuclear missiles proceed a short distance away in Geneva from the START sessions. There will be no attempt to link the two by the end of this year. To try this would likely overburden the INF sessions, which face their own unique deadline. The INF round technically has no termination date. But the US is scheduled to deploy its cruise and Pershing missiles if no pact is reached by year-end. The administration isn't saying precisely when deployment will start, beyond the general phrase ''before the end of the year,'' but no missile parts or assemblies are to arrive in West Germany, for instance, until after a Bundestag debate on November 21. The host governments themselves will announce the actual deployment timing.
Failing an INF pact this year, it appears likely the INF and START talks might be merged in 1984, itself a progressive step.
In the broad scope of the arms talks, progress remains possible. On the Soviets' side, East European countries are showing their own nervousness over the two superpowers' arms standoff.
Former President Nixon, in his forthcoming book (''Real Peace: A Strategy for the West'') suggests ''the time is ripe for a deal'' with Andropov. ''While he has taken the reins of the Soviet Government, he is not yet firmly in the saddle. He needs a foreign policy initiative,'' Nixon writes.
Mr. Reagan's latest proposals suggest a flexibility that the Soviets can meet with a positive response if they want to.